These were already mentioned
1. no script
2. key scrambler
Here are 2 more
3. flashblock
4. BetterPrivacy
If you're using anything other than Firefox, you have to switch. I'm no fanboy, but FF has the biggest number of useful plug-ins of any browser. Yes, Opera may be faster and more robust and lot's of people Luuuuuvvvv Safari. I've even heard good things about Chrome. But it's the difference between buying a Model T (in any color you want - as long as it's black) and buying a Lexus LS450.
Some plug-ins (like KeyScrambler) will also work in IE, but most are exclusive to FF. And even if you only DL those 4, they are INVALUABLE. No shit.
But every good defensive strategy has multiple layers.
Start with an intelligent NAT, SPI router. It doesn't matter what that stuff is, any good (Linksys, DLink, Trendnet, Netgear) router will do.
If you want to get fancy, dust off an old 3/486 and install smoothwall or something similar. But plan on learning more about networking that you'd ever want to know.
Next, you should have 2 on-the-fly scanners and at least one firewall. I like Avira (
www.avira.com) security suite and Malwarebytes. Avira is not free (but you can use it for 30days). Malwarebytes is free for the scanner, but not for the version that is memory resident and scans on the fly.
Avira is supposed to have very good heuristics, which means that it can identify potential (aka 0-day) threats. Other programs are mainly database driven, so if you're the first sucker to find a virus, worm, trojan, etc., uhhhh, congratulations?
Zonealarm used to be good and may still be and they have a free version. Last I heard, they were going to license Sophos' heuristics, but I don't know if that ever happened.
Third, update your OS daily. Have it set to automatically DL and install at least the latest security patches. This is so easy to do and is the last thing most people think of.
Lastly, eliminate as many threats as possible at the point of contact, i.e., your browser. The odds of someone hacking your machine by exploiting a vulnerability in the OS are pretty much zero if you keep it up to date. The real risk is with Java and Java script, ActiveX apps and the like. But unless you have NoScript, there is no way to allow/deny these on a site by site basis - at least not easily.